Oil shale deposits are found in many parts of the world, making the fuel available for generating power on a large scale. One of the problems in designing power plants operating on oil shale is caused by the large variations in heating value of shale mined from the same field. Specifically, in some fields, the heating value may vary in the range from less than about 600 Kcal/Kg to about 2000 or more Kcal/Kg depending on the strata from which the oil shale is extracted. Accommodating such variation complicates the design of the combustor for the oil shale.
A combustor designed for use with oil shale is usually constructed so that a set rate of feed of the oil shale in Kg/hr. into the combustor will produce rated heat in Kcal/hr. when the heat value of the oil shale has some average value. For example, a combustor to produce 1 million Kcal/hr. using oil shale having a heating value of 1000 Kcal/Kg would be designed to have a throughput of 1000 Kg/hr. of oil shale. If, during actual operation, the heating value of the oil shale would drop to 600 Kcal/Kg, the throughput would have to be increased to 1667 Kg/hr. to maintain the same combustor heat output. This is physically not possible for a combustor of fixed size.
To accommodate variations in heating value, oil shale power plants segregate stockpiles of oil shale according to heating value, and select shale from different stockpiles by appropriate selection to obtain a blend with a substantially constant heating value. This approach complicates the operation of such power plants by reason of the requirement for blending, and the large storage areas for the different types of oil shale.
It is therefore an object of the present invention to provide a new and improved method of and apparatus for efficiently combusting oil shale, or other low grade solid fuels, which reduces the complexity of power plants operating on such low grade fuels.